Packaged microelectronic assemblies, such as memory chips and microprocessor chips, typically include a microelectronic substrate die encased in a plastic, ceramic, or metal protective covering. The die includes functional devices or features, such as memory cells, processor circuits, and interconnecting wiring. The die also typically includes bond pads electrically coupled to the functional devices. The bond pads can be coupled to pins or other types of terminals that extend outside the protective covering for connecting to busses, circuits and/or other microelectronic assemblies.
As the size of microelectronic device packages decreases to allow the packages to fit into more compact electronic products (such as mobile phones and laptop computers), the distances between adjacent functional devices and between adjacent interconnecting wires decreases. As these distances decrease, the likelihood for capacitive coupling between adjacent structures increases, which can impair or reduce the maximum performance of the packaged microelectronic device.
One approach to decreasing the capacitance between neighboring wires within the die is to reduce the dielectric constant of the solid material between the wires. For example, polyimides (having a dielectric constant of 3.5) have been used to replace silicon dioxide (having a dielectric constant of 4). A more substantial reduction in the dielectric constant is obtained by replacing the solid insulating material typically positioned between layers of the wiring with a gas, such as air. For example, U.S. Pat. No. 5,891,797 to Farrar and U.S. Pat. No. 5,324,683 to Fitch et al. disclose a process for building successive layers of wiring on a semiconductor substrate by temporarily supporting the wires with sacrificial filler material, and then removing the filler material from around the wires by etching or a plasma process to form suspended “air bridges” that conduct electrical signals from one part of the device package to another. The wires can be formed in the filler material using a dual damascene process, such as is disclosed in U.S. Pat. No. 4,962,058 to Cronin et al. The support material can include a resist material, as disclosed in U.S. Pat. No. 5,593,926 to Fujihiri, that can be removed by etching processes (such as the processes disclosed in U.S. Pat. No. 4,561,173 to Te Velde) or evaporative processes (such as the processes disclosed in U.S. Pat. No. 5,408,742 to Zaidel et al.). U.S. Pat. Nos. 5,891,797; 5,324,683; 4,962,058; 5,593,926; 4,561,173; and 5,408,742 are herein incorporated in their entirety by reference.
It can be shown that the maximum unsupported link of an air bridge in an integrated circuit is governed by the following equation:L=4√{square root over (32Eδh2/5p)} or ≈1.6(Eδ/p)1/4h1/2
where L=the unsupported bridge length
E=the modular elasticity of the bridge material
δ=the maximum allowable deflection of the bridge
ρ=the density of the bridge material
h=the vertical thickness of the bridge
As microelectronic devices become smaller, the thicknesses of the bridges and the distances between adjacent bridges also become smaller. To prevent the bridges from sagging into each other, the maximum unsupported length of each bridge decreases. For example, if the bridge is made of an aluminum copper silicon alloy (which has a module of elasticity of 71 GPa and a density of 2.79 Mg/m3), has a maximum allowable deflection of 5,000 angstroms (including a safety factor), and a thickness of 10,000 angstroms, the maximum unsupported bridge length is approximately 1.6 millimeters. If the maximum allowable deflection is decreased to 2,500 angstroms, and the bridge thickness is reduced to 5,000 angstroms, the maximum unsupported bridge length is approximately 1 millimeter. If the maximum allowable deflection is further decreased to 1,500 angstroms, the maximum allowable unsupported length is approximately 0.6 millimeters. Because current chips typically measure over 1 centimeter along an edge, it becomes increasingly difficult to reduce the thickness of the bridges and the spacing between bridges without supporting the bridges at such frequent intervals that the benefits of unsupported bridge segments (e.g., the reduced dielectric constant of the material adjacent to the bridge) are lost.
Furthermore, as the bridge thickness (and therefore the cross-section of the conductive line forming the bridge) decreases, the resistivity of the wire forming the bridge increases. One approach to addressing this drawback is to reduce the bulk resistivity of the wire, for example, by replacing aluminum alloy wires with copper wires. However, copper has a significantly greater density than aluminum and aluminum alloys, and therefore has only 85% of the unsupported bridge length of an aluminum or aluminum alloy conductor.
Another problem with conventional air bridge designs is that the air adjacent to the wires typically has a lower thermal conductivity than the solid material it replaced. Accordingly, it can be more difficult to transfer heat from the packaged microelectronic device. As a result, the microelectronic device may be more likely to overheat, which can reduce the life expectancy and/or performance level of the device.